The clock extraction is used to provide a central timing point of eye for the purpose of setting up synchronism between a transmitter and a receiver, and correctly discriminating equalized signals.
In a conventional optical communication system, a clock is obtained in accordance with an electric signal converted from a light signal by using an optical to electric (O/E) converter, for instance, as described below.
On pages 85 and 86 of "the technology for PCM communication" published in August of 1976 by Sanpo Shuppan, a self-timing extraction method is described. In this method, an electric signal is obtained from a signal light coded in NRZ coding to be propagated via a transmission line by using an O/E converter and a clock component of the electric signal is emphasized by a differential turn-back circuit, so that an unnecessary frequency component is removed to provide a clock signal by using a band-pass filter having a high Q value.
On pages 106 to 109 of the above described literature, a method using a phase locked oscillator is described. In this method, a phase locked loop (PLL) comprising a phase detection circuit, a low band-pass filter, and a voltage controlled oscillator (VCO) is used, wherein the phase detection is carried out in the phase detection circuit between an input signal and an output signal of the VCO, and an output signal of the phase detection circuit is supplied to the low band-pass filter to be fed back to the VCO, so that a clock signal which is synchronous with the input signal is obtained.
On pages 510 to 551 of "Electronics Letters, Vol. 28, No. 5, February, 1992", an optical signal processing system in which high speed signal processing is carried out to cope with recently developed high speed optical communication with a data rate greater than 10 Gb/s is described. This system comprises a PLL circuit utilizing an optical correlative detection in a semiconductor optical amplifier. In this system, a pulse coded signal light of a clock frequency f and a clock light of a repetition frequency (f+.DELTA.f) which is an output light of a clock light generator driven by a clock obtained from an oscillation frequency f of a VCO which is superposed with a frequency .DELTA.f are supplied to the optical amplifier, in which a gain of the signal light is modulated in accordance with gain saturation by the clock light, so that a component of the frequency .DELTA.f which is a correlation between the signal and clock lights is generated in an output signal light of the optical amplifier. Consequently, a clock light which is locked in phase to the signal light is obtained by detecting a phase fluctuation of the .DELTA.f component, and feeding the detected phase fluctuation back to the VCO.
On the other hand, the processing of electric signals makes it difficult to demultiplex a time sequential signal from a signal light which is multiplexed with time sequential signals of a transmission capacity of more than several ten Gb/s, due to the limitation in operation speed.
For this reason, it is required to process signals in a light region, as explained in the below literatures.
On pages 962 to 964 of "Electronics Letters, Vol. 27 No. 11, May, 1991", a four-wave mixing switch in which a signal light and a clock light are supplied to an optical fiber, and four-wave mixing occurs in the optical fiber dependent on the overlapping degree of pulses between the signal and clock lights is described.
On pages 962 to 964 of "Electronics Letters, Vol. 26 No. 14, July, 1990", a non-linear optical fiber loop mirror in which signal lights are propagated through an optical fiber in both directions, and a clock light is propagated through the optical fiber in one of the both directions, so that the signal light propagating in the same direction as the clock light is subject to cross phase modulation, and the signal light thus modulated and the signal light propagating in the opposite direction are interfered to provide switching operation is described.
On pages 1186 to 1198 of "IEEE Journal of Selected Areas in Communications, Vol. 6, No. 7", an optical Kerr switch is described. In the optical Kerr switch, linearly polarized signal and clock lights having an angle of 45.degree. therebetween are supplied to an optical fiber, and the polarization of the signal light is rotated to provide switching operation by 90.degree. by using the different of cross phase modulations between a component of the signal light parallel to the polarization of the clock light and a component thereof orthogonal thereto.
On pages 340 to 341 of "Electronics Letters, Vol. 24 No. 6, March 1988", a Mach-Zehnder type optical switch is described. In this optical switch, one of two divided signal light is supplied to a non-linear optical medium along with a clock light, an the signal light is shifted in phase in the non-linear optical medium by cross phase modulation, so that the cross phase modulated signal light and the remaining one of the two divided signal light are interfered to provide switching operation.
However, the clock extraction apparatus using the self-timing method or the phase locked oscillator has a disadvantage in that a operation frequency is several tens GHz at most, and it is difficult to extract a clock from a ultra-high speed signal light of, for instance, 100 Gb/s.
The optical PLL circuit using the semiconductor optical amplifier has a disadvantage in that an operation frequency is limited to be several tens GHz at most by the semiconductor optical amplifier. Further, one more clock light generator is required to generate a clock light of a frequency f locked in phase to the signal light, because the repetition frequency of the obtained clock light is (f+.DELTA.f) for the reason that the frequency .DELTA.f is superposed on the output signal of the VCO.
The optical time-division demultiplexing apparatus has a disadvantage in that the phase lock between the signal and clock lights is required to necessitate the conventional optical clock extracting apparatus. This results in large size, high cost and large power consumption.